A rediscovered 1970s surf film has revealed the life of 18-year-old Trudie Adams before she went missing from Sydney's northern beaches 40 years ago.

Key points:

18-year-old Trudie Adams has been missing since 1978

Nobody has ever been charged for her suspected murder

Her body has never been found

The film, called Highway One, was unearthed by the ABC's investigative team while making Barrenjoey Road — the ABC's TV series and podcast that is investigating the teenager's disappearance.

Adams was last seen hitchhiking home from the Newport Surf Club on 24 June, 1978. Her disappearance sparked New South Wales' biggest missing person search at the time, and her suspected murder has been investigated by police four times.

Highway One was filmed one year before Adams disappeared, and features her in several scenes, including at Avalon Beach with her friends.